1. Field of the Invention
The present invention is directed to a method for manufacturing a film resistor to be used as a thermal conductivity detector, particularly in gas analyzers.
2. Description of the Prior Art
It is known in the art to use precision resistors as resistance thermometers. In such devices, a thin platinum layer is applied onto an electrically non-conductive carrier, such as glass or ceramic. The thin platinum layer may be applied by employing a high-vacuum evaporation or cathode sputtering method, whereby the layer may cover the entire surface of the carrier or may cover only a partial region thereof. This is disclosed in German Patent DE-PS No. 828930.
It is likewise known in the art, as shown in German published application DE-OS No. 2507731, to employ precision platinum resistors composed of an insulating carrier formed from a material having a greater coefficient of thermal expansion than platinum at temperatures between 0.degree. and 1000.degree. C. Such precision resistors, however, are restricted with respect to the types of materials which may be utilized as the carrier.
Thin-film resistors, in contrast to the precision resistors used in resistance thermometers, are constructed having extremely high resistance values and optimally low temperature coefficients. In the manufacture of such thin-film resistors, it is known, as disclosed in German published application DE-OS No. 2019091, to apply the resistance layers by cathode sputtering in an argon-atmosphere for the embedding of foreign atoms.
A method for manufacturing a precision film resistor for use in resistance thermometers is also disclosed in German patent DE-PS 2558752. The precision film resistor disclosed therein is composed of an insulating carrier and a thin resistance layer of platinum which is disposed on the carrier by cathode sputtering in an atmosphere containing an inert gas. The following steps are suggested:
(a) employment of a krypton-oxygen mixture or xenon-oxygen mixture to produce an inert gas atmosphere; PA1 (b) application of a countervoltage to the carrier; PA1 (c) post-tempering the sputtered-on resistant layer in an oxidizing atmosphere at a temperature between 700.degree. and 1200.degree. C.
None of the above-noted precision resistors are suitable for integration in gas analyzers which are utilized, for example, in analyzing the exhaust gas during the development of reciprocating motors by the automobile industry. Such exhaust gases are extremely reactive and readily penetrate the known protective layers. Consequently, after such precision resistors are exposed to such gasses for a relatively short duration, the resistance layer is destroyed.
It is therefore an object of the invention to set forth a method for manufacturing a precision film resistor suitable for high operating temperatures (i.e., exhibit a high intrinsic temperature stability, particularly at temperatures above 150.degree. C.), which is insensitive to aggressive gases and which can be manufactured in a particularly economical manner.